Solid electrolyte, method of preparing the same, and secondary battery including the same

A solid electrolyte comprising:

• an inorganic lithium ion conductive film;
• and
• a porous layer on a surface of the inorganic lithium ion conductive film,
• wherein the porous layer comprises a first porous layer and a second porous layer, and the second porous layer is disposed between the inorganic lithium ion conductive film and the first porous layer, and
• wherein the first porous layer has a pore size which is greater than a pore size of the second porous layer,
• wherein the first porous layer comprises a first inorganic lithium ion conductor substituted with about 2% to about 100% protons, and the second porous layer solid electrolyte comprises a second inorganic lithium ion conductor substituted with about 0.01% to about 20% protons, based on the total number of protons and lithium ions.